This invention relates to navigational systems for use in aircraft and other means of transportation utilizing transmission and receiver devices for the handling of radio and radar information, this information being used to develop data needed to navigate the aircraft. This data may include direction of travel, distance to or from fixed reference points, and the measurement of azimuth.
In general aviation, the standard navigational system uses VHF OMNI Range network, commonly referred to as VOR, wherein the pilot of the aircraft can determine his bearing with reference to the location of selected VOR transmitters. The transmitters at various geographical locations operate on different VHF carrier frequencies, but each conveys a pair of 30 Hz signals which, when received by the aircraft, provide an indication of the angular position of the aircraft relative to the particular transmitter by means of the phase relationship of the signals. Ordinarily, one of the 30 Hz signals is established as a phase reference signal and is frequency modulated upon a subcarrier having a nominal frequency of 9,960 Hz, the subcarrier in turn being amplitude modulated upon the VHF carrier. The VOR station also transmits a CW radio signal through a rotating directional antenna such that a 30 Hz AM signal is provided in space at the recieiving aircraft. This AM signal has a varying phase for each degree of bearing away from a reference bearing chosen for the station. The reference bearing is due north. An aircraft receiving both the AM and FM signals contains a phase detector which detects the phase difference between the 30 Hz modulation carried by the AM and FM signals. This phase difference is measured in degrees and is a direct indication of the bearing of the aircraft relative to the reference phase of the VOR station.
Similarly, many aircraft are equipped with instrument landing systems (ILS). Such ILS systems are utilized as an aid to landing for instrument approach when the ceiling and visability are low. ILS as it exists today includes three basic ground station signals, which are (1) the localizer signals (LOC); (2) the glide slope signals (GS); and (3) the marker signals. Functionally, these elements provide lateral, vertical and longitudinal (distance) information respectively by which it is possible to navigate an aircraft from a prescribed approach altitude to a lower level close to the runway. From this lower level, usually 200 feet above the runway, the pilot is expected to see the runway or a system of approach lights so that the actual landing is accomplished by visual reference to the runway.
The "localizer" (LOC) defines a vertical plane of radiation created by generated signals which permits the aircraft to align with the centerline of the runway. The localizer signal has two modulation frequencies, 90 Hz and 150 Hz. The desired course is defined by equal amplitudes of each.
"Glide slope" (GS) is a signal at the right place and the proper angle from the initial approach plane to the plane of the runway which will enable the aircraft to make a proper landing. A typical glide slope angle is normally 3.0.degree., which is an angle generally satisfactory for all present day fixed wing aircraft. Two modulation frequencies, 90 Hz and 150 Hz are employed. The desired glide slope angle is defined by equal amplitudes of each. In the aircraft's receiver, the 150 Hz signal output would predominate below the glide slope and a 90 HZ output would predominate above the glide slope. A discriminator therefore is capable of putting out a signal which can be displayed to the pilot designating whether he is below or above the desired glide path. Again, as with the localizer, in the past special purpose equipment dedicated to processing glide slope information has typically been used in aircraft. This equipment has been connected to its own antenna which is capable of receiving 90 Hz and 150 Hz signals. A carrier frequency of 330 MHz is used which permits the use of a low antenna structure at the airport.
The ever continuing demand, however, to reduce the weight and space taken up by equipment carried on an aircraft makes it desirable to reduce the weight and size of the navigational unit avionics. The various individual systems comprising a total navigational unit carry within themselves a duplication of electronic components. While it may be conceivable to utilize common equipment which can be dedicated to handling of more than one type of signal, the actual implementation of such a system becomes quite complicated involving information rates of incoming signals, duty cycles of the shared components, and sequencing and control of the shared system.
An object of this invention is to provide a shared navigational unit wherein components common to more than one of the standard navigational systems are utilized on a shared basis having a plurality of dedications.
Another object of this invention is to provide a logic and control scheme for such a shared component navigational unit wherein a timing and control sequence permits processing of a plurality of information without detrimental delay to any one process and without the loss of any information.
A further object of this invention is to provide such a unit wherein the shared processing components are operated in such a manner that a beat frequency or false signal pertubation is not created.
An even further object of this invention is to provide such a unit where said time shared operation of said common components operation is multiplexed, where false signal outputs are inhibited during switching, and where detrimental stored historical information is destroyed during switching.